[RFC v2 PATCH 03/10] vfio/pci: Add helper to look up PFNs for DMABUFs

[Matt Evans <mattev@meta.com>]

Add a helper, vfio_pci_dma_buf_find_pfn(), which a VMA fault handler
can use to find a PFN.

This supports multi-range DMABUFs, which typically would be used to
represent scattered spans but might even represent overlapping or
aliasing spans of PFNs.

Because this is intended to be used in vfio_pci_core.c, we also need
to expose the struct vfio_pci_dma_buf in the vfio_pci_priv.h header.

Signed-off-by: Matt Evans <mattev@meta.com>
---
 drivers/vfio/pci/vfio_pci_dmabuf.c | 102 +++++++++++++++++++++++++----
 drivers/vfio/pci/vfio_pci_priv.h   |  19 ++++++
 2 files changed, 108 insertions(+), 13 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
index 44558cc2948e..63140528dbea 100644
--- a/drivers/vfio/pci/vfio_pci_dmabuf.c
+++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
@@ -9,19 +9,6 @@
 
 MODULE_IMPORT_NS("DMA_BUF");
 
-struct vfio_pci_dma_buf {
-	struct dma_buf *dmabuf;
-	struct vfio_pci_core_device *vdev;
-	struct list_head dmabufs_elm;
-	size_t size;
-	struct phys_vec *phys_vec;
-	struct p2pdma_provider *provider;
-	u32 nr_ranges;
-	struct kref kref;
-	struct completion comp;
-	u8 revoked : 1;
-};
-
 static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
 				   struct dma_buf_attachment *attachment)
 {
@@ -106,6 +93,95 @@ static const struct dma_buf_ops vfio_pci_dmabuf_ops = {
 	.release = vfio_pci_dma_buf_release,
 };
 
+int vfio_pci_dma_buf_find_pfn(struct vfio_pci_dma_buf *vpdmabuf,
+			      struct vm_area_struct *vma,
+			      unsigned long address,
+			      unsigned int order,
+			      unsigned long *out_pfn)
+{
+	/*
+	 * Given a VMA (start, end, pgoffs) and a fault address,
+	 * search the corresponding DMABUF's phys_vec[] to find the
+	 * range representing the address's offset into the VMA, and
+	 * its PFN.
+	 *
+	 * The phys_vec[] ranges represent contiguous spans of VAs
+	 * upwards from the buffer offset 0; the actual PFNs might be
+	 * in any order, overlap/alias, etc.  Calculate an offset of
+	 * the desired page given VMA start/pgoff and address, then
+	 * search upwards from 0 to find which span contains it.
+	 *
+	 * On success, a valid PFN for a page sized by 'order' is
+	 * returned into out_pfn.
+	 *
+	 * Failure occurs if:
+	 * - The page would cross the edge of the VMA
+	 * - The page isn't entirely contained within a range
+	 * - We find a range, but the final PFN isn't aligned to the
+	 *   requested order.
+	 *
+	 * (Upon failure, the caller is expected to try again with a
+	 * smaller order; the tests above will always succeed for
+	 * order=0 as the limit case.)
+	 *
+	 * It's suboptimal if DMABUFs are created with neigbouring
+	 * ranges that are physically contiguous, since hugepages
+	 * can't straddle range boundaries.  (The construction of the
+	 * ranges vector should merge such ranges.)
+	 */
+
+	const unsigned long pagesize = PAGE_SIZE << order;
+	unsigned long rounded_page_addr = address & ~(pagesize - 1);
+	unsigned long rounded_page_end = rounded_page_addr + pagesize;
+	unsigned long buf_page_offset;
+	unsigned long buf_offset = 0;
+	unsigned int i;
+
+	if (rounded_page_addr < vma->vm_start || rounded_page_end > vma->vm_end)
+		return -EAGAIN;
+
+	if (unlikely(check_add_overflow(rounded_page_addr - vma->vm_start,
+					vma->vm_pgoff << PAGE_SHIFT, &buf_page_offset)))
+		return -EFAULT;
+
+	for (i = 0; i < vpdmabuf->nr_ranges; i++) {
+		unsigned long range_len = vpdmabuf->phys_vec[i].len;
+		unsigned long range_start = vpdmabuf->phys_vec[i].paddr;
+
+		if (buf_page_offset >= buf_offset &&
+		    buf_page_offset + pagesize <= buf_offset + range_len) {
+			/*
+			 * The faulting page is wholly contained
+			 * within the span represented by the range.
+			 * Validate PFN alignment for the order:
+			 */
+			unsigned long pfn = (range_start >> PAGE_SHIFT) +
+				((buf_page_offset - buf_offset) >> PAGE_SHIFT);
+
+			if (IS_ALIGNED(pfn, 1 << order)) {
+				*out_pfn = pfn;
+				return 0;
+			}
+			/* Retry with smaller order */
+			return -EAGAIN;
+		}
+		buf_offset += range_len;
+	}
+
+	/*
+	 * If we get here, the address fell outside of the span
+	 * represented by the (concatenated) ranges.  Setup of a
+	 * mapping must ensure that the VMA is <= the total size of
+	 * the ranges, so this should never happen.  But, if it does,
+	 * force SIGBUS for the access and warn.
+	 */
+	WARN_ONCE(1, "No range for addr 0x%lx, order %d: VMA 0x%lx-0x%lx pgoff 0x%lx, %d ranges, size 0x%lx\n",
+		  address, order, vma->vm_start, vma->vm_end, vma->vm_pgoff,
+		  vpdmabuf->nr_ranges, vpdmabuf->size);
+
+	return -EFAULT;
+}
+
 /*
  * This is a temporary "private interconnect" between VFIO DMABUF and iommufd.
  * It allows the two co-operating drivers to exchange the physical address of
diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h
index 27ac280f00b9..5cc8c85a2153 100644
--- a/drivers/vfio/pci/vfio_pci_priv.h
+++ b/drivers/vfio/pci/vfio_pci_priv.h
@@ -23,6 +23,19 @@ struct vfio_pci_ioeventfd {
 	bool			test_mem;
 };
 
+struct vfio_pci_dma_buf {
+	struct dma_buf *dmabuf;
+	struct vfio_pci_core_device *vdev;
+	struct list_head dmabufs_elm;
+	size_t size;
+	struct phys_vec *phys_vec;
+	struct p2pdma_provider *provider;
+	u32 nr_ranges;
+	struct kref kref;
+	struct completion comp;
+	u8 revoked : 1;
+};
+
 bool vfio_pci_intx_mask(struct vfio_pci_core_device *vdev);
 void vfio_pci_intx_unmask(struct vfio_pci_core_device *vdev);
 
@@ -110,6 +123,12 @@ static inline bool vfio_pci_is_vga(struct pci_dev *pdev)
 	return (pdev->class >> 8) == PCI_CLASS_DISPLAY_VGA;
 }
 
+int vfio_pci_dma_buf_find_pfn(struct vfio_pci_dma_buf *vpdmabuf,
+			      struct vm_area_struct *vma,
+			      unsigned long address,
+			      unsigned int order,
+			      unsigned long *out_pfn);
+
 #ifdef CONFIG_VFIO_PCI_DMABUF
 int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
 				  struct vfio_device_feature_dma_buf __user *arg,
-- 
2.47.3